Performance specifications for three dimensional sensors vary widely, with a figure of merit often determined based upon specific requirements. Many applications impose simultaneous demands of rapid data rates for 100% coverage of the entire inspection area yet accuracy and precision measured in microns or perhaps finer. In other cases, very high speed inspection may be required in areas of interest, with other regions ignored.
With increasing trends toward miniaturization, manufacturers of microelectronic assemblies and miniature parts often have requirements for very fine measurement or defect detection capability for automatic three-dimensional optical inspection equipment. Tradeoffs exist between the field of view, spot size, depth sensitivity, and measurement speed. Because measurement requirements are demanding, a system which is optimized for imaging only areas of interest and can avoid "dead" areas can provide effective solutions for process monitoring where specific regions of interest are monitored, provided the system is also effective for 100% inspection.
For inspection of microelectronic assemblies and other miniature parts, the data rate is a key parameter, and the attainable inspection speed is dependent upon the pattern to be inspected. FIG. 1 illustrates a case where the inspection sites are "randomly" arranged, the inspection task may be height and width measurements of all circuit traces, solder height, and component dimensions. Defects like extra material, missing material, and misplaced devices must also be detected. Such defects are indicated by dashed "boxes" in FIG. 1. There is very little "dead area" which can be ignored by the inspection system. Suppose the region in FIG. 1 represents a small section (say 0.1".times.0.1") of a dense patterned wafer. If the inspection system utilizes a typical dynamic focus probe about 1000 data points per second will be acquired. Suppose that 2.5 .mu.m.times.2.5 .mu.m samples (which can be coarse for inspection of such devices) are taken over the entire surface. The resulting inspection time for a 6".times.6" object is 1.3 months! Point triangulation sensors currently available operate at about 10,000 points per second and reduce the inspection time to a few days, and with special designs can achieve data rates of about 100 KHz. Although it is possible to increase the data rate of point probes to video rates (1000 fold improvement), the requirements for the necessary motion mechanism would be unwieldy and perhaps completely impractical.
On the other hand, a significant fraction of all microelectronic assemblies have inspection sites which are regularly arranged in a series of rows and columns, often along the sides of a rectangle or square. FIGS. 2 through 5 illustrate several cases of importance. Such patterns are found over a large scale, lead widths from a few mils (1 mil=0.001") to tens of mils, and ball (and bump) diameters from 10 microns (bumped die) to about 1 mm (BGA 225). The interconnect patterns are similar at these scales. However, the reduced coverage requirement does not guarantee that the data rate requirement (square inches/second, sites/second, etc.) can be substantially reduced. The minutes allowed for a video rate system to perform 100% inspection of a wafer may be reduced to a few seconds for 100% inspection of lead or bonding sites (electrical interconnections). Inspection requirements for such regions usually correspond to a three dimensional measurement of the lead (ball or pad) geometry, coplanarity, and width. These measurements may be in the production line (1 chip at a time) or "in-tray" (FIG. 6) to inspect the three-dimensional structure of leads and solder pads (FIG. 5) of ICs after placement. Improper placement (in 3D) can create defects which are not evident until after soldering thereby greatly increasing board scrap and rework costs. This application requires high resolution and very high data rates, approximately 3-4 seconds per chip, for inspecting all the sites on a board without affecting production throughput. Other applications (requiring a finer scale) in microelectronics having similar requirements include three dimensional inspection of wirebond and bumped wafer patterns.
A 3-D line scan sensor which has capability for 100% inspection (figure of merit in square inches/second or sites/second), yet can be controlled to optimize inspection for a regular, repetitive pattern (figure of merit typically sites/second) can significantly increase the utility of the inspection system. Accuracy and high speed, which are mutually conflicting parameters, are both required to maximize the return on investment of inspection equipment and widespread applicability of the inspection equipment in the industry.
3-D line-scan triangulation-based sensors which approach video rates, as described in U.S. Pat. No. 5,024,529, reduce the time for 100% inspection of the above sample to several minutes which is acceptable for many facilities. This patent is hereby incorporated by reference herein.
Of all known methods for 3-D imaging, triangulation provides the most practical method for a high speed-accuracy product (figure of merit). However, unlike point triangulation sensors, laser line scan systems do not have rotational symmetry. A scan line may be viewed in either the longitudinal or transverse directions (depends upon preference), in either case the height is being derived from a geometric relationship along a position sensing axis. Although it is possible to derive mathematical transformations which will provide exact registration between height values obtained at arbitrary viewpoints, the implementation of the hardware and software to support the requirements requires substantial additional effort.
Although imaging speed is crucial, accuracy is an issue which is equally important. Triangulation-based sensors require off-axis arrangements for illumination/viewing and a necessary extended instantaneous field of view (IFOV) along the depth sensing axis. Certain part geometries create difficulties with occlusion and associated spurious reflections.
FIG. 7 illustrates problems occurring when inspecting the leads of an IC. In FIG. 7, the compound problem of occlusion and secondary specular reflection is illustrated when an IC is imaged with the surrounding area being flat (without three dimensional structure), and the problem will be further compounded if additional surfaces (i.e. walls of a tray containing parts) are added. The IC leads are metallic, and may have a smooth, shiny appearance. In this case, obtaining useful data with both sensors at this orientation requires a relatively narrow viewing angle (&lt;20 degrees). However, anomolies can still exist in certain geometries because one sensor may be partially occluded and the second is corrupted by secondary reflections. Both problems "disappear" if the same leads are viewed from the orthogonal direction. The secondary specular reflection is not received by either sensor, and no obstruction is present which will adversely affect inspection capability. Hence, an appropriate choice of imaging geometry makes a significant difference in capability.
FIG. 8 illustrates a somewhat different but common scenario for BGAs, circularly symmetric bumps or bonding pads. Detector 1 receives a weak signal due to the surface orientation (shading) while detector 2 receives a strong signal (in error) resulting from secondary reflections from the shiny bonding pad. The more complex problem of dealing with a curved surface, shiny finish, and partial occlusion must be addressed with the use of multiple sensors and non-linear pattern recognition and multi-channel estimation techniques, where both the intensity and range information are used to eliminate readings in error. This approach is described in detail in copending U.S. patent application Ser. No. 08/245,864.
A versatile 3-D triangulation-based imaging system providing both high measurement speed and improved accuracy should result in increased application of automated 3-D measurement.
Stern et al., U.S. Pat. No. 5,371,375, discloses a method and system for scanning IC leads in a tray using a two-pass approach resulting from a limited sensor field of view (FOV). The first pass uses a sensor to locate features from which the necessary translation and rotation is computed for a second pass. The 3-D sensor then measures the lead dimensions. The small field of view is a requirement if CCDs or photodiode arrays are to be used, because of readout limitations (60 fields/second standard video). If a small array (say 64.times.64) is used, the data rate can be increased to roughly 4000 lines/second maximum which is within typical requirements for lead inspection. However, the TV sensor technology is not well matched for high speed 100% inspection. For example, the line rate would be decreased to about 60 lines/second if standard arrays are used for wide field scanning.
Chen et al., U.S. Pat. No. 5,118,192, discloses a triangulation-based scanning system constructed to have projection and viewing axes independently pivotable, thereby providing flexibility for inspection and measurement of irregular objects. The 3D scanning method can also be employed for inspection of IC leads in a tray, solder joints and other regularly arranged patterns and provides for multiple axes of illumination and viewing. The pivoting action also allows for triangulation-based measurement along orthogonal rows and columns, provided that a rotary mechanism is utilized for 90.degree. dynamic head rotation.
Tokura, U.S. Pat. No. 5,192,982, discloses a laser scanning method for inspection of soldered leads. The problems with directional reflectance (asymmetry) and resulting invalid data are recognized and the illumination and viewing directions are chosen accordingly. The system is designed primarily for examining the appearance of solder fillets which are smooth and have a shiny surface finish.
Maruyama et al., U.S. Pat. No. 5,200,799, discloses a 3-D laser scanning system for inspection of parts packaged on PCBs which utilizes a pair of position sensitive detectors and an image processing system, including a circuit for multi-sensor pre-processing. The illustrated embodiment includes a polygon laser scanning system and a large lens for wide FOV coverage, up to 100% of the circuit board. The inertia of the rotating mirror prevents "region of interest" or random access scanning. The scanning method includes movement of the circuit board along a Y axis and a scan beam along an X axis. This typical scanning method restricts the viewpoint of the system leading to consideration of issues shown in FIG. 7. Also, a tradeoff exists between the wide field coverage, resolution, and effective data rate and is further illustrated herein.
Ray, U.S. Pat. No. 5,058,178, describes an imaging system for detecting missing or defective solder bumps. The system uses dark field illumination and a TV camera. 3D data is not acquired so the system is not capable of providing coplanarity measurements, and is highly sensitive to the wide variations in reflectance and background variations which are found for metallic surfaces. The system can be successfully used for detecting missing material or gross soldering defects at relatively fast data rates.
Merry et al., U.S. Pat. No. 4,700,045, describes the use of an acousto-optic deflector and programmable scan width for following a random seam on a surface. A tradeoff can be made between the scan speed of the stage and the number of points collected for each scan line. The nature of (commercially available) acousto-optic deflectors allows for "windowing" capability as the deflection is proportional to an applied frequency which, in turn, can be programmed through computer control. These solid-state deflectors are preferred. Galvonometer-based deflectors also allow for random beam deflection but require moving mechanical parts and additional circuitry or other means for stabilization and position correction.